The present invention relates to a method and apparatus for converting a digital video image to a higher aspect ratio format at a decoder by line dropping. The invention is particularly suitable for use with digital video data corresponding to the MPEG-2 standard.
The transmission of digital video data via broadband communication systems such as cable television or satellite television networks has become increasingly popular. Digital decoders, e.g. set-top boxes, are provided in consumers' homes for use in receiving the digital video signals and processing the signals in a format that is suitable for display on a television, video display terminal, or the like. However, the processing and memory storage requirements of such set-top boxes is significant and must be kept as low as possible in order to maintain the commercial feasibility of the network.
In particular, in many cases it is desirable to provide a capability at the decoder for converting a digital video image having an aspect ratio, e.g., 4:3, that is sized for conventional television screens, to a higher aspect ratio, e.g., 16:9. A 16:9 aspect ratio is commonly used for filming motion pictures that are intended to be shown on screens in movie theaters. This aspect ratio allows a greater range of horizontal motion in the image.
However, the 4:3 aspect ratio has prevailed in television screen design and frame formats in the existing television transmission standards, including Phase Alternation Line (PAL) and National Television System Committee (NTSC). Accordingly, to display a conventional NTSC or PAL signal in a 16:9 format on a television screen, the image appears with black regions above and below the active video lines.
Moreover, decompression of digital video signals, such as those conforming to the MPEG-2 standard, requires the use of an external memory, such as Dynamic Random Access Memory (DRAM), to provide frame buffers for the different picture types (e.g., I-, P- and B-pictures), and to provide a Video Buffer Verifier (VBV) buffer. The buffer arrangement is generally optimized for handling the MPEG-2 Main Profile at Main Level requirement. The MPEG-2 standard also requires the VBV buffer to be at least 1.75 Mbit. With these limitations, and the memory arrangement existing in current video decompression processors, there is no additional memory space or memory bandwidth available to support 16:9 aspect ratio conversion by letterbox at a decoder.
Accordingly, it would be desirable to provide a system for converting the aspect ratio of a digital video image at a decoder without requiring additional memory space or memory bandwidth. The system should be compatible with existing digital video standards, including MPEG-2, and existing television transmission formats, including PAL and NTSC.
In a particular embodiment, the system should provide an architecture and methodology which uses only a 16 Mbit EDO DRAM operating at 27 MHz (with a 32-bit data bus), or a SDRAM operating at 54 MHz (with a 16-bit data bus) for converting a 4:3 aspect ratio image to a 16:9 aspect ratio image. However, the system should provide other conversions to larger aspect ratios as required.
The system should be compatible with both field mode (e.g., interlaced scan) and frame mode (e.g., progressive scan) video.
The present invention provides a system having the above and other advantages.